1. Field of the Invention
The invention relates to the field of precision electrical discharge machining (EDM) of metal components, and provides a method and apparatus for confirming the continued accurate positioning of one or more electrodes on an EDM machining head and for detecting out of position or misaligned electrodes.
2. Description of the Related Art
In EDM an electrode, or a number of electrodes carried on a machining head, is moved into close proximity to a metal or other electrically conductive workpiece, and a series of electrical sparks or discharges generated between the or each electrode and the workpiece. The material of the workpiece is vaporized, and by advancing the machining head a hole or holes can accurately be machined into the workpiece. The shape of the hole is dictated by the shape of the electrode. For example a round wire electrode will create a hole of circular section. A metal strip electrode of curved or arcuate section will create a slot-shaped hole of corresponding curved or arcuate section. The only practical limit is presented by the shape of the electrode.
EDM machining is used for the very high precision creation of multiple holes in workpieces. For example the blades and vanes of jet engine turbines may be machined by this method. Such components are provided with multiple air holes through which cooling air passes when the component is used in a jet engine. The cooling air flow is critical, since without it the component would deform or even melt at the temperatures established in a jet engine in use. To that end the size shape and angular orientation of each air hole is specified within close tolerances. Each blade or vane may have tens or hundreds of air holes machined in it before it can be used, and different air holes in the same component frequently have different specifications to ensure the proper operation of the component in use. The angular orientation is controlled by having the machining head carried on a numerically controlled robot arm having at least three degrees of freedom of movement.
A problem which has long been experienced in EDM fabrication of components has been the high cost of quality control and of rejection of inaccurately machined components. The entire machining process is carried out under computer control and in theory should therefore be consistent and accurately reproducible. With that automation and computer control, however, comes an inevitable loss of continuous visual inspection of the workpiece during machining. The machining takes place in a closed cabinet, and when it is complete the workpiece must be checked for the total accuracy of the drilling.
A very common cause of drilling inaccuracies is the deformation of the EDM electrode or electrodes. If that electrode is a fine wire of diameter 0.2 to 0.5 mm, for example, it may easily become bent in use so that its axis is no longer aligned with the machining head axis. Even a small bend will mean that the robot arm places the tip of the wire electrode in marginally the wrong starting position on the workpiece for the commencement of a machining operation, but as the machining progresses and the hole is formed deeper into the workpiece, so the electrical discharge extends along one side of the electrode which is no longer advanced along its own axis. The result is a machined hole that is both misaligned and deformed. Users are currently discarding, during the quality control inspection, components worth £1000 or more.